@static_sports
One of your major problems is that you are using (in your own words) "thick cardboard envelopes" -- these envelopes are too rigid to be used with the eBay Standard Envelope (ESE) label. The ESE is intended to be used with a #6 or #10 "plain white envelope" (PWE), which can be flexible enough to be bended nearly in half when traveling at high speed through the USPS roller systems.
I've been successfully using the ESE for several years, mailing stamps, trading cards, and stickers; and I just use a simple #6 PWE. I trim 2 pieces from material constructed from either cereal boxes or 12 pack soda cartons -- these trimmed pieces should fit snugly inside the envelope. In between these 2 strips, add the item to be mailed, using either a penny-sleeve or a glassine envelope -- nothing which will be too rigid to be kicked out of the USPS high speed automatic roller systems.
Then seal the envelope, affix the ESE label tightly to the front, and mail it in the "Metered Mail" slots inside in the USPS lobbies -- the ESEs will then be treated like all other USPS contracted business metered mail, and assigned an acceptance number at the time that they are received at the next USPS Distribution Center.
Do NOT hand your ESEs over to USPS counter employees, or USPS drivers. And do NOT use the USPS "blue boxes" or the mailing services from private businesses, as all of these may delay the acceptance scan at the USPS Distribution Centers (or the scan may never even be received). Use the "Metered Mail" slot in many post office lobbies.
In addition, as @lacemaker3 pointed out to you, you are also attempting to mail items using the ESE which do not qualify for ESE labels -- please re-read the ESE product restrictions --
"As of Nov 2023, four additional categories are now eligible to use eBay standard envelope, expanding the service to eight categories. See the full list of category paths.
- Patches
- Stickers & Decals
- Greeting Cards
- Seeds
- Trading Cards
- Coins & Currency
- Postcards
- Stamps"
So some of your shipments are very likely being kicked out of the USPS automatic high speed mailing system for being "non-machinable." Eventually, these will either be shipped to your buyers and labeled "postage due" (which will not make your buyers happy!), or returned to you and labeled "insufficient postage." But don't hold your breath -- I've been waiting for the return of a similar item since the end of July!
USPS considers the ESE to be just another piece of contracted business metered mail, which USPS handles every day in numbers totaling in the hundreds of thousands. Metered mail does not use standard "tracking" numbers -- they receive an initial acceptance number at the time that they are received at the next USPS Distribution Center (and NOT at local post offices!), and possibly a "Delivered" number at the time that they are received at the post office matching the ZIP Code of the buyer's residence address -- but NOT the residence address itself. Actual delivery to the buyer's address usually takes several more days, or longer.
So, some very simple solutions to the problems which you are encountering:
(1) Do not use "thick cardboard envelopes" to mail items using the ESE -- the envelope MUST be VERY flexible!
(2) Review the items which you are attempting to mail via the ESE -- if those items aren't on the list, they can not be mailed using the ESE.
(3) Use the "Metered Mail" slots in the USPS lobbies for faster and more efficient acceptance and delivery.
The ESE can be an efficient and money-saving tool for eBay sellers, once some of the very basic rules are fully understood and regularly observed.
Good luck!
